The present invention relates to a planetary gearing for an automatic transmission, and more particularly to a speed multiplication technique of a planetary gearing.
There have been known two approaches to a speed multiplication of a planetary gearing for an automatic transmission which provides at least three forward speeds and one reverse speed. One approach which is disclosed in JP 59-117943 A is to add an auxiliary planetary gearing to a planetary gearing which has four forward speeds with an overdrive as the fourth speed. The auxiliary planetary gearing serves as an overdrive mechanism when the main planetary gearing is in the fourth speed to provide another overdrive as a new fifth speed.
Another approach is disclosed in JP 50-155868 A. According to this known patent specification, an auxiliary planetary gearing is added to a planetary gearing which has three forward speeds with a direct drive as the third speed. The auxiliary planetary gearing serves as a reduction mechanism when the main planetary gearing is shiftable among the first, second and third speeds, while it shifts to a direct drive state thereof when the main planetary gearing is in the third speed. This results in providing new first to third speeds which have speed ratios larger than those of the first to third speeds which were provided by the main planetary gearing alone.
Let it be considered that the second approach is applied to a planetary gearing which has four forward speeds with an overdrive as the fourth speed. Applying this approach to the planetary gearing of the above-mentioned type results in addition of an auxiliary planetary gearing in such a manner that the auxiliary planetary gearing serves as a reduction mechanism when the main planetary gearing is in any one of the first to fourth speeds, while it is locked when the main planetary gearing is in the fourth speed. In the main planetary gearing, there is at least one rotary member which rotates at an increased speed relative to a revolution speed of an input shaft during the overdrive state. The first to fourth speeds are frequently used in the overall planetary gearing. This causes an increase in the frequency at which the main planetary gearing is in its overdrive state where some rotary members of the main planetary gearing rotate at the increased speed relative to the revolution speed of the input shaft. This results in increased friction loss of bearings supporting the rotary member. Thus, the endurability of the bearings have to be improved.
An object of the present invention is to provide a speed multiplication technique of a planetary gearing which has n (n: an integer not less than four) forward speeds such that the first or lowest speed of the overall planetary gearing has a speed ratio larger than a speed ratio of the first speed provided by the main planetary gearing alone.
A specific object of the present invention is to provide a planetary gearing for an automatic transmission, which planetary gearing provides n+1 speeds (n: an integer not less than four) and includes a main planetary gearing having n speeds and an auxiliary planetary gearing operatively coupled with the main planetary gearing such that the lowest or first speed of the overall planetary gearing has a speed ratio larger than a speed ratio provided by the first speed of the main planetary gearing alone, and a ratio between a speed ratio of any one forward speed and a speed ratio of the next adjacent higher another forward speed is not greater than a ratio between the speed ratio of the another forward speed and a speed ratio of the next adjacent higher forward speed.